The present invention relates to a bipolar transistor, including a semiconductor body having a cathode-side surface and an anode-side surface, and at least one insulated gate electrode. The transistor has the following features. The semiconductor body has a central region with a predetermined doping concentration and of a first conductivity type. The central region borders on the cathode-side surface of the semiconductor body. Bordering on the cathode-side surface, at least one gate region is provided which borders on the central region. The gate region is of the second conductivity type and has a higher doping concentration than the central region. In the gate region, a source region is provided which borders on the cathode-side surface. The gate electrode is seated on an insulating layer applied on the cathode-side surface and covers the gate region. Between the anode-side surface and the central region is provided an anode region of the second conductivity type which has a higher doping concentration than the central region. Between gate region and source region, a shunt is provided.
Such a semiconductor structural element has been described, for example, in the article "COMFET-The Ultimate Power Device" in the journal Solid State Technology, November 1985, pages 121 to 128. On the cathode side it is structured like a power MOSFET, however, on the anode side it has a region which is of the opposite type of conductivity from the central region. Consequently, it has a thyristor structure and is provided on the cathode side with a shunt which increases the so-called latching current known from the thyristor in such a way that it is not reached during operating conditions of the structural element. Both types of charge carriers participate in current transport, as is the case with the thyristor but in contrast to the power MOSFET. On the one hand, this has the advantage of a low conducting state dc resistance but does, on the other hand, have the disadvantage of a storage charge which delays switching off.
The storage charge was diminished according to different suggestions, for example, by introducing substances forming recombination centers or through disturbance sites generated by irradiation. A further suggestion is that between the central region and the anode region a buffer region is inserted which is of the same type of conductivity as the central region, but has a higher doping concentration. See, for example, the article "Power Control" in the journal Electronic Design, Jan. 12, 1984, pages 248 and 249.
With the described measures, or with a combination of both measures, a reduction of the storage charge, and with it a reduction of the switch-off time, can be achieved. It is the goal of the invention to achieve, with simple means, a further decrease of the storage charge and a shortening of the switch-off time.